Hitherto, as a surveillance camera having a dehumidification capability, a surveillance camera in which a dehumidifying element is attached to a camera housing has been known. For example, Japanese Patent Laid-Open No. 2005-260923 (pages 4 to 10, FIG. 1) discloses such a surveillance camera. The dehumidifying element has a configuration in which a polymer electrolyte membrane is sandwiched between an anode electrode film and a cathode electrode film. In the surveillance camera of the related art, a drive voltage is applied between the two electrode films of the dehumidifying element to dehumidify the interior of the camera housing.
In the surveillance camera of the related art, when a drive voltage is applied between the two electrode films, water molecules in the camera housing are electrolyzed into hydrogen ions and oxygen ions on the anode (moisture-removing-side) electrode film. In this case, the polymer electrolyte membrane serves as a hydrogen ion exchange membrane, and the hydrogen ions move to the cathode (moisture-discharging-side) electrode film. On the cathode electrode film, the hydrogen ions react with oxygen in the air, and water molecules are formed. Thus, the interior of the camera housing is dehumidified by moving the moisture in the air on the anode side of the dehumidifying element into the air on the cathode side.
However, in the surveillance camera of the related art, the drive voltage applied to the dehumidifying element is fixed irrespective of the humidity in the camera housing. Therefore, when the humidity in the camera housing is high, it takes relatively long time to lower the humidity in the camera housing.
For example, FIG. 7 is a graph for explaining how the surveillance camera of the related art described above is dehumidified. As shown in FIG. 7, when the power supply for the dehumidifying element is turned on at time t0, a large number of hydrogen ions pass through the polymer electrolyte membrane of the dehumidifying element when the humidity in the camera housing is high, and hence a large value of drive current I0 is applied to the dehumidifying element. Since the drive voltage applied to the dehumidifying element is fixed in the surveillance camera of the related art, the drive current through the dehumidifying element gradually decreases at a substantially constant rate as the humidity in the camera housing decreases. After the drive current applied to the dehumidifying element decreases to the smallest current value Imin at time t7, the dehumidifying element will then keep operating at the smallest current value (standby current). As described above, in the surveillance camera of the related art, it takes relatively long time to lower the humidity in the camera housing when the humidity therein is high.
To dehumidify the interior of the camera housing in accordance with the humidity therein, it is conceivable to use a hygrometer that measures the humidity in the camera housing. In this case, however, it is necessary to provide a space for attaching the hygrometer in the camera housing, and an extra cost of the hygrometer is also required.